This application claims priority of Taiwan patent Application No. 091105630, filed on Mar. 22, 2002.
1. Field of the Invention
The invention relates to an amplifying circuit, more particularly to an amplifying circuit for limiting amplitude of output signal supplied to a speaker.
2. Description of the Related Art
An electronic device for multimedia applications, such as a notebook computer, is usually provided with an audio codec that receives and integrates various types of sound sources (such as wave, MIDI, CD, line IN, beep, phone line, etc.), and an amplifying circuit to amplifying the integrated audio signals for reproduction by built-in loudspeakers or external loudspeakers and earphones.
Referring to FIG. 1, a conventional amplifying circuit 6 is shown to include an operational amplifier 61, and two resistors (Z1, Z2). The operational amplifier 61 has an inverting input end 62 for receiving an input signal (VIN) via the resistor (Z1), a grounded non-inverting input end 61, and an output end 64. The resistor (Z2) is connected between the inverting input end 62 and the output end of the operational amplifier 6. The operational amplifier 61 receives the input signal (VIN) via the resistor (Z1) and generates an output signal (VOUT) at the output end 64. It is known that the relationship between the input and output signals (VIN, VOUT) is             V      OUT        =                  -                              Z            2                                Z            1                              ⁢              V        IN              ,
wherein the ratio of the output signal (VOUT) to the input signal (VIN) is       -                  Z        2                    Z        1              ,
which is otherwise known as the amplifier gain. The negative sign indicates that the input and output signals (VIN, VOUT) have an inverted relation. Thus, the amplitude of the output signal (VOUT) depends on a ratio of impedances of the resistors (Z2, Z1) such that when the gain is fixed, the larger the input signal (VIN), the larger will be the output signal (VOUT).
However, different types of sound sources have different amplitudes. For example, MIDI generally has a relatively large amplitude. Further, due to the absence of a standard amplitude during recording of audio signals, a load, such as loudspeakers and earphones, (not shown), which is connected to the output 64 of the operational amplifier 61, may be damaged when the output signal (VOUT) supplied by the conventional amplifying circuit 6 exceeds a fixed rated power of the load.
Therefore, the object of the present invention is to provide an amplifying circuit capable of limiting amplitude of an output signal supplied to a speaker.
According to the present invention, an amplifying circuit capable of limiting the amplitude of an output signal supplied to a speaker comprises:
a first resistor unit;
an amplifier having a first input connected to the first resistor unit, and an output, the amplifier receiving an input signal at the first input via the first resistor unit and generating the output signal at the output for supplying to the speaker;
a second resistor unit having a first end connected to the first input of the amplifier, a second end connected to the output of the amplifier, and at least one control input, the second resistor unit having a variable equivalent impedance that is controlled by a control signal received at the control input; and
a feedback control unit having an input end connected to the output of the amplifier so as to receive the output signal therefrom, and at least one output end connected to the control input of the second resistor unit, the feedback control unit being operable so as to provide the control signal to the control input of the second resistor unit when the output signal does not fall within a voltage range defined by a predetermined low voltage and a predetermined high voltage to adjust the variable equivalent impedance of the second resistor unit.